The production of hydrocarbon fluids from a subsea reservoir often results in oil, gas and water being transferred concurrently through a common pipeline. In the instance of producing marginal, subsea, satellite fields, a large diameter pipeline is utilized. This pipeline, sometimes several miles in length, is installed between the remote field and the production platform or facility. Often, the selection of the pipe diameter results in two phase slug, or alternating gas-liquid flow passing through the pipeline.
Producing operations to enhance recovery from a hydrocarbon holding reservoir sometime necessitates the facility of gas lift. This factor, although productive, is known to induce intermittent gas liquid flowing conditions in the pipeline.
A reasonably constant liquid flow to processing equipment at a remote offshore structure normally requires an intermediate buffer apparatus, which treats the two phase flow. The latter can be a large vessel or tank which functions to absorb the liquid slugs that will provide steady liquid feed during the passage of the intermittent gas bubbles. The current trend of the oil industry to move into subsea production systems has mandated that subsea slug catchers be adapted to the arduous conditions of offshore operations.
The ordinary slug catcher, for use in a liquid-vapor stream usually embodies traditional design concepts. Such units, however, have proven to be relatively expensive both from the point of view of fabrication, and from the consideration of installing such units at offshore locations. For example, in North Sea operations, the installation of any form of subsea equipment can be a costly and dangerous operation. This is due primarily to the adverse weather conditions prevalent for much of the working season.
Where a marine platform or structure has been fixed in place at the sea floor, it is normally designed to accommodate a predetermined number of wells, and a certain volume of production. Such a structure is not normally provided with the necessary producing equipment to receive multi-phase fluid streams from one or more satellite wells. Often the marine platforms are not designed for expansion beyond their immediately contemplated capacity.
As a general practice, the platform structure and its ancillary equipment are designed to accommodate only original equipment with perhaps room for minor expansion. When it becomes necessary to add additional production equipment or otherwise to increase capacity, the platform's ability to sustain or support such equipment may be exceeded. Such a circumstance could endanger the safety and efficiency of the entire unit as well as personnel.
In accordance with the invention, to facilitate the production and handling of multi-phase hydrocarbon flows between a subsea well and an offshore producing platform, a pipeline carries the hydrocarbon flow to the marine platform. The latter flow will normally comprise a liquid hydrocarbon stream with intermittently spaced gas bubbles interposed in the liquid flow.
When this pressurized, two phase flow arrives at the production platform, it is discharged into an upstanding slug catcher to achieve a smooth flow. The primary function of the latter is basically to stabilize the flow to the processing equipment. It additionally, however, acts as a gas-liquid separator. In the present arrangement, the slug catcher comprises a first upright column to initially separate the fluid into liquid and gaseous phase. A second upright column receives residual liquid for additional separation.
Operationally, the gaseous segment of the hydrocarbon fluid stream will rise through the fluid separator. This upward movement is due to the weight of the gas as well as to the elevated pressure at which it arrives at the slug catcher. The liquid phase on the other hand will be passed into the adjacently positioned production riser. In said member, the liquid segment is further separated and the residual liquid pumped into the processing equipment at the platform's deck.
The slug catcher's upright members are incorporated into the existing platform structure beneath the work deck. Said added members are further maintained in a substantially upright position by being slidably registered in the platform's conductor guides which might otherwise go unused.
Operationally, there is provided a method for continuously treating or pretreating a multi-phase, hydrocarbon containing fluid stream having liquid and varporous phases. The stream is initially separated whereby to remove a major part of the vaporous phase.
The liquid phase is then transferred to a riser which holds an amount of the liquid phase, to allow a relatively quiescent separation of additional vapor. The two vaporous streams are then combined for further treatment. Concurrently, the substantially gas-free liquid is conducted from the reservoir.
It is therfore an object of the invention to provide a low cost subsea production system for a hydrocarbon fluid source such as subsea wells, including means to separate the produced fluid into liquid and vapor phases.
A further object is to provide a marine structure which includes liquid hydrocarbon processing equipment, and self-supporting separating means which handles a multi-phase stream of the fluid from offshore wells.
A still further object is to provide an elongated slug catcher for an offshore marine platform wherein the slug catcher is self-supported on the sea floor and is laterally braced in an upright disposition by conductor guides positioned within the platform structure.